The present invention relates to the adjustment of the distance and spacing between the print head and the platen in a printer, particularly a matrix printer, wherein the print head proper is mounted on a carriage which, in turn, runs on a rail system including, particularly a pair of shafts or round bars, whereby one of the bars is being provided as a rail for running the print head carriage in the horizontal along the platen while the other bar runs and holds the carriage in the vertical, but permits movement of the carriage towards and away from the platen, whereby the first bar itself is adjustable in some fashion transverse to its extension, for varying the distance between that bar and the carriage on it and the platen.
Eccentric cam-like devices are used for purposes of adjusting the spacing between the print head and the platen, which feature permits the operating personnel to adjust the distance between the print head proper and the paper running through the printer on the platen so as to adjust basically the appearance of the printed characters. If multiple layers of paper are fed through for purposes of producing a plurality of copies such adjustment may be advisable in order to adapt the printer to specific types of paper or the like. Such adjustment however is destroyed, for example, when the ribbon has to be changed and a new ink ribbon cartridge has to be inserted or if the printer is used subsequently with a different type of print medium, differently thick paper, a different number of copies or the like. Also, the threading-in of the new print medium, particularly in case of an endless sheet, may often require some retraction of the print head so that in each case a new adjustment of the print head is necessary. Moreover, during extensive use of the printer it has to be observed that in the case of a matrix printer the print needles exhibit certain wear. In such a situation, a readjustment of the print head is necessary. This will be particularly true if for reason of excessive wear the needles, or the print head as a whole, are to be exchanged. This way, the head does not just have to be receded from the platen but has to be removed entirely. It is clear, therefore, that one cannot possibly rely on some initial optimization in adjusting the distance between the print head and the platen and/or the print medium but frequently situations arise in which a readjustment is necessary.
Eccentric cam devices working for adjusting the distance between the print head and the platen are shown, e.g. in U.S. Pat. No. 4,178,106. It has to be stated, however, that this particular arrangement is meaningful only for a platen which is non-round, i.e. has a flat printing surface. This is the element 9 in that patent. In addition, however, the adjusting device of that patent is insufficient, particularly concerning latching in some form of the adjusting lever to a null position, or null readjustment or adjustment to a null position, is rather imprecise. Moreover, the eccentric adjustment devices of that patent seem to be inadequately guided as far as the adjustment proper is concerned.
A somewhat more precise guiding structure is disclosed in U.S. Pat. No. 4,609,294, corresponding to German printed patent application No. 30 14 823. Herein, a particular frame is proposed being linearly guided within a printer housing. This frame is adjustable towards and away from the platen. The structure disclosed in this patent requires, in addition, a device for resting and latching the position as between the frame and the printer housing. Thus, the latter patent avoids the problems of imprecise guiding of the adjusting structure, but is, however, encumbered by a high cost aspect which, for economic reasons, is not always justifiable.
Independently from the foregoing, the German utility model (petty patent) Pat. No. 7140524, proposes that the print head is not adjustable but the platen itself is. On each of the two ends a separate adjustment device is provided. This particular printer then includes essentially three movable but interconnected groups of elements. There is, first, a pivotable holder, second, a lever element which responds to this adjustment movement, and, third, a fine or high precision coupling element. Therefore, there is a multiple of parts, all being adjustable which means that the tolerances inherent in each of these components compound additively. This results in inaccuracies between the various parts rendering the adjustment and the restorability of the particular adjustment position rather imprecise. Particularly the parallelity of the adjusting movement of the two devices at the two ends of the printing platen drum is subject to differences based on these tolerances.